[MM-36872] add threading headers to notification emails (#18232)

* [MM-36872] Adding headers for mail threading

* MM-36872 Add message id and mail threading headers

* MM-36872 Add message id and mail threading headers

* MM-36872 Add msgId, inreplyto, references headers

* Fixing comment issues

* Applying gofmt lint fixes

* Applying gofmt lint fixes

* MM-36872: Updates required from merge.

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Martin Kraft <martin@upspin.org>

app/email/mocks/ServiceInterface.go

@@ -302,13 +302,13 @@ func (_m *ServiceInterface) SendLicenseUpForRenewalEmail(_a0 string, name string
 	return r0
 }
 
-// SendMailWithEmbeddedFiles provides a mock function with given fields: to, subject, htmlBody, embeddedFiles
-func (_m *ServiceInterface) SendMailWithEmbeddedFiles(to string, subject string, htmlBody string, embeddedFiles map[string]io.Reader) error {
-	ret := _m.Called(to, subject, htmlBody, embeddedFiles)
+// SendMailWithEmbeddedFiles provides a mock function with given fields: to, subject, htmlBody, embeddedFiles, messageID, inReplyTo, references
+func (_m *ServiceInterface) SendMailWithEmbeddedFiles(to string, subject string, htmlBody string, embeddedFiles map[string]io.Reader, messageID string, inReplyTo string, references string) error {
+	ret := _m.Called(to, subject, htmlBody, embeddedFiles, messageID, inReplyTo, references)
 
 	var r0 error
-	if rf, ok := ret.Get(0).(func(string, string, string, map[string]io.Reader) error); ok {
-		r0 = rf(to, subject, htmlBody, embeddedFiles)
+	if rf, ok := ret.Get(0).(func(string, string, string, map[string]io.Reader, string, string, string) error); ok {
+		r0 = rf(to, subject, htmlBody, embeddedFiles, messageID, inReplyTo, references)
 	} else {
 		r0 = ret.Error(0)
 	}